using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main(string[] args)

    {

        string[] data = { "1", "2", "3", "5", "6", "7", "4" };

​

        var list = new List<string>(data);

​

        for (var i = 0; i < list.Count; i++)

        {

            if (list[i] == "5")

            {

                list.Insert(i, "");

                i++;

            }

        }

​

        data = list.ToArray();

​

        Console.WriteLine(string.Join(",", data));

    }

}